砂岩核磁共振响应模拟及受限扩散

本文运用随机游走方法模拟了砂岩储层中流体的核磁共振(NMR)响应及其受限扩散现象.通过改变数字岩心的分辨率模拟生成不同孔隙尺寸的砂岩,研究了不同孔隙尺寸砂岩饱含水时流体扩散系数随扩散时间的变化关系,同时模拟了砂岩饱和单相流体和两相流体的NMR响应;研究了流体的受限扩散系数与横向弛豫时间T2的关系,分析了表面弛豫率和胶结指数对润湿相流体受限扩散系数线位置的影响,并将其用于解释砂岩储层的D-T2分布.结果表明:孔隙流体的扩散系数会随扩散时间的增加而逐渐减小并趋于定值.随着岩石孔隙尺寸的减小,受限扩散现象越明显,受限扩散对岩石NMR响应的影响也越大.润湿相流体受限扩散系数线的位置受岩石胶结指数和表面弛豫率的影响较大.由于润湿相流体扩散系数减小,导致D-T2分布中润湿相流体信号偏离其自由扩散系数线,需要利用流体的受限扩散系数线准确识别D-T2分布中的润湿相流体.

Simulation of NMR responses in sandstone and restricted diffusion

Nuclear magnetic resonance (NMR) responses and restricted diffusion phenomenon of the fluids in sandstone were simulated by the Random-Walk method. Sandstone of different pore sizes was generated by changing digital cores' resolution, permitting to study how the fluid diffusion coefficient changes with time when the sandstone of different pore sizes were saturated with water, and to simulate the NMR responses of the either single or two phase fluids in the sandstone. Then the relationship between the fluid restricted diffusion coefficient and the T₂ relaxation time was researched, and the influence on restricted diffusion phenomenon of wetting phase fluid posed by the surface relaxivity and cementation index of the sandstone was analyzed. Finally the analysis results were used to interpret the D-T₂ distributions of the sandstone. It was concluded that the diffusion coefficient of fluid gradually decreases and tends to be constant with increase of diffusion time. The smaller pore size of the rock, the restricted diffusion phenomenon is more obvious, and the restricted diffusion has a greater influence on NMR responses. The position of the restricted diffusion coefficient line of wetting phase fluid is affected by the rock cementation index and surface relaxivity. The wetting phase fluid signal in the D-T₂ distributions deviates from the unrestricted diffusion coefficient line due to the decreasing diffusion coefficient of the wetting phase fluid, hence the restricted diffusion coefficient line of fluid should be used to recognize the wetting phase fluid in D-T₂ distributions.